Daydream:
If you really feel that if you are hosting, you have to invite her, I agree with a previous suggestion of not hosting. You can't be "blamed" for not inviting her over to someone else's house.

Of course, I feel that you don't have to invite her and that, if you didn't, the blame would lie with her and her family.

I feel that would be the best option and when you let her know, it should be clear that it is a decision based on her and her family's behavior. They made that choice, not you. In fact, you are doing them a favor. So:

"Sister, I understand that it makes you and your family uncomfortable to not discuss politics and not swear during holiday visits. We've discussed this before and you've made it clear that refraining from these things is not an option for you during these times. For everyone's comfort, it would be best if we celebrated holidays separately from now on. Maybe we could get together for lunch, just you and I (whenever)?"

Pen^2:
I agree with PPs. The fact that she's blatantly refused to behave politely means she won't listen to you unless it suits her. She can't even refrain from foul language? Really? Wow. I hope she is never around children at any point in her existence, then. ::) I'm sure she can control herself, unless she has some sort of disorder or verbal tick. She just doesn't want to. She may enjoy offending others--who knows? But it's irrelevant. If the host asks that you behave politely and you can't/won't, then too bad. Time to go. That's all there is too it.

If you have to have her there because you live in the same place, have it at a restaurant or ask someone else to host. If you have to invite her because otherwise it would upset your mother, for example, then that's harder. If a relative says you must offend other people by having a known rude person who refuses to be respectful to the host (you) present, then I'd recommend growing a strong spine ASAP. Being polite to all your guests trumps following the rude wishes of one or two relatives. Not doing what people ask isn't rude if they're asking something unreasonable. Asking that you have a person present who is knowingly rude to others is not reasonable. "Keeping the peace" at the expense of others doesn't fly when one of the others is your spouse. Once you get married, your spouse comes first. Not inviting someone because they are rude and upset other guests is not rude. If people get offended at that, then that's their own problem.

If you think it might possibly work, then give her one last chance. But be ready to escort her out the moment she blows it.

kudeebee:
She and her family are making the majority of your guests uncomfortable during family get together. You have talked to her about this before, many times, and she still does it. And why shouldn't she, as there have not been any consequences for her actions.

It sounds like maybe it is time to do separate family get togethers rather than combining both sides.

pierrotlunaire0:
Sis, I have to be honest with you. I am dreading Thanksgiving because I know that your language will be blue and your politics will make everyone else uncomfortable. Should I have separate celebrations? Should I not invite you? I cannot have you here when you act like that and although I have asked you before, it's always back to the same behavior.